1. Response of the Earth’s environment to solar radiative forcing
Ingrid Cnossen
British Antarctic Survey
Image credit: NASA

2. Outline This lecture Intro to atmosphere structure and processes
Absorption of solar radiation
Atmosphere composition
Energy balance
Ionization
Conductivity
Low latitude currents
Next lecture
Effects of variations in solar radiative forcing (examples)
High-latitude currents
Coupling to magnetosphere

3. Vertical structure of the atmosphere
D layer
F2 layer
F1 layer
E layer
Thermosphere
Mesosphere
Stratosphere
Troposphere 2 3 4 5 6 7
400
350
300
250
200
150
100 50
600
800
1000
Electron density (log10 cm-3)
Temperature (K)
Height (km)
Low
High
Air density
scale height 3

5. The yellow indicates absorption, scattering/reflection by the atmosphere. Main absorber at larger wavelengths (>1000 nm) is H2O (water). CO2 (carbon-dioxide) absorbs around 2000 nm. The sharp dip near nm is due to O2 (oxygen). Ozone absorbs in the UV range around 250 nm in the stratosphere. Shorter wavelengths get absorbed at higher altitudes in the middle and upper atmosphere.

6. Absorption of solar radiation
Balance between
Intensity of incoming solar radiation
Atmosphere density

7. Atmospheric absorption cross-sections
X-ray
EUV
FUV
middle UV
near UV
From Cnossen et al. (2007)

8. What happens with absorbed solar energy?
EUV photon
Photo-ionization
Photo-dissociation
Excitation
Fast electron
(photo-electron)
Slow ion (He+)
He atom
EUV photon
Heating
Chemical reactions
Radiative loss (cooling) 8

9. Transformation of radiative energy
From Fomichev (JASTP, 2009)
LTE applies only in a dense atmosphere, where collisions are frequent, so energy can be considered to be instantly converted to heat (very quick recombination; quick release of energy from excited states).
In a less dense atmosphere, collisions are not as frequent, and energy stored in an excited state or in chemical potential energy is much longer lived, and can be transported over considerable distance.
LTE radiative processes
non-LTE radiative processes
non-LTE chemical processes
non-LTE kinetic processes
LTE = local thermodynamic equilibrium

10. Energy Thresholds for Processes
Species
Dissociation
(Å)
(eV)
Ionization H He O O2 N2 NO
2423.7
1270.4
1910
5.11
9.76
6.49
911.75
504.27
910.44
1027.8
796
1340
13.6
24.58
13.62
12.06
15.57
9.25
From Heubner et al., Astrophys. Space Sci., 195, 1-294, 1992

11. Neutral atmosphere compositionAr O2 N2
Important minor species:
NO (nitric oxide)
CO2 (carbon dioxide)
O3 (ozone)
heterosphere
Density per cubic meter. At 100 km we have about 10^20, which is about 10^14 cm^-3
turbopause (balance between molecular and eddy diffusion)
homosphere

12. Absorption of solar radiation within the atmosphere
Image credit: John Emmert/NRL

13. Energy balance in the atmosphere
planetary balance
zonal mean absorbed solar radiation
zonal mean outgoing infrared radiation
From Mlynczak (2000)

14. Winds in the thermosphere

15. Radiative energy terms in the middle atmosphere
IR = infrared cooling
CP = chemical heating
Sol = solar heating
LTE
non-LTE
From Fomichev (JASTP, 2009)

16. Radiative energy terms in the middle atmosphere
What latitude?
Tropical latitudes, because heating exceeds cooling!
IR = infrared cooling
CP = chemical heating
Sol = solar heating
LTE
non-LTE
From Fomichev (JASTP, 2009)

17. Middle atmosphere radiative heating and cooling
solar heating
infrared cooling
H2O (>18.5 μm)
CO2
CO2 O3
H2O (<18.5 μm)
O2 (<205 nm) O3
O2 (>205 nm)
H2O
CH4
LTE
non-LTE
From Fomichev (JASTP, 2009)

18. Thermosphere heating and cooling rates (solar min)
Roble et al. (1987)
O2 absorption
exothermic chemical reactions
total neutral heating
total neutral cooling
NO 5.3 μm emission
O3 absorption
CO2 15 μm emission
collisions with ions/electrons
quenching of O(1D)
atomic oxygen cooling
molecular thermal conduction
auroral particle precipitation
eddy thermal conduction
O recombination
Joule heating

19. Photochemistry in the upper atmosphere
From Richards (JGR, 2011)

20. Ionosphere composition
Major species: O2+, NO+, and O+
Molecular ions dominate < ~150 km; approximate photo-chemical equilibrium
O+ dominant > ~200 km

21. Conductivity of the ionosphere
𝑁_𝑒

22. Solar quiet (Sq) current system
Winds drive currents
Observations from l’Aquila
Currents perturb the magnetic field

23. Summary Solar activity affects…
Heating and cooling processes throughout the atmosphere
Pressure gradients and winds
Composition
Ionization
Conductivity
Currents in the ionosphere
These processes also interact with each other!